Grasses mixture-planting rather than fertilization depresses soil microbial diversity in an alpine artificial grassland

Purpose There is an increasing need to establish artificial grasslands for managing sustainable pastures under global environmental change. Many studies are attempting to clarify the ecological factors that control soil biodiversity, which regulate ecosystem functioning. However, the effect of plants and soil fertility on soil microbial diversity, particularly in alpine artificial grasslands, remains unclear. Methods We examined the importance of mixture-planting (two-species mixtures vs. monocultures of three grass species) and the addition of chemical fertilizer on the diversity of soil bacteria and fungi in a 14-year experimental grassland on the Tibetan alpine habitat. Results Mixture-planting pronouncedly depressed not only the richness and diversity of soil bacteria but also the richness of soil fungi, with the decrease in soil fungal richness partly buffered by the effect of fertilization. The decline in soil bacterial and fungal richness was partly explained by a decline in soil total nitrogen following mixture-planting. Neither mixture-planting nor fertilization significantly impacted the community composition of soil microbes, although mixture-planting and the decreased soil nitrogen significantly altered the relative abundance of a few dominant bacteria. Conclusion By taking advantage of the relatively long-term experiment, our study provides robust evidence that mixture-planting of grasses can decrease soil microbial diversity, potentially by reducing soil total nitrogen or the effect of weeds in artificial grassland. Appropriate fertilization should be considered to compensate the loss of soil nutrients and to ensure the soil function of the artificial grassland.